Methods are known whereby copper and small amounts of an alloying metal, such as magnesium or aluminum, are co-sputtered onto a substrate having oxide on at least a portion of its surface (See, e.g., U.S. Pat. No. 6,037,257 and U.S. Pat. No. 6,160,315). Either the wafer is held at an elevated temperature during deposition or the sputtered film is annealed without the wafer being exposed to atmosphere. Due to the high temperature, the alloying metal diffuses to the surface. If a surface is exposed to a low partial pressure of oxygen or contacts silicon dioxide, the magnesium or aluminum forms a thin stable oxide. The alloying metal oxide encapsulates the copper layer to provide a barrier against copper migration, to form an adhesion layer over silicon dioxide, and to act as a seed layer for the later growth of copper, for example by electroplating.
Likewise, U.S. Pat. No. 6,066,892 and U.S. Pat. No. 6,387,805 describe methods whereby a Cu-Mg or Cu-Al alloy is sputter deposited and used as a superior wetting or seed layer prior to the deposition of a substantially pure copper layer.
U.S. Pat. No. 6,479,389 describes variants of the above methods whereby sputtering or physical vapor deposition (PVD) of a copper alloy film is then followed by chemical vapor deposition (CVD) or electrochemical deposition (ECD) of a pure copper film. In another variant, CVD or ECD deposition of a pure copper film is then followed by the sputter deposition of a copper alloy film. For both embodiments, a separate thermal treatment is used to improve the copper alloy formation and to enhance the corrosion resistance and electromigration resistance of the resulting copper alloy.
Such conventional methods have significant drawbacks in that sputtering is a non-conformal process so that varying thicknesses of the alloying element containing copper are deposited inside high aspect ratio (ratio of feature height to feature width) trenches and vias. This can make it very difficult to uniformly introduce a dopant and can lead to uneven distributions of the alloying element within patterned features. Although the composition of the alloying element can be well controlled in the sputter target source material, once sputtering takes place, the concentration of the alloying element becomes difficult to control inside high aspect ratio features since sputtering is a line of sight process and the various constituents of the target source material can have differential sputter rates. Too high a concentration can lead to high film resistivities, whereas too low a concentration can prevent the beneficial aspects of the alloying element. In addition, the conventional processes typically require substantial thermal budget, which is not preferred when used in conjunction with low dielectric constant (i.e., low-k materials) especially when the dielectric constant is <3.0 due to the temperature stability constraints of these low-k materials.
In U.S. Pat. No. 6,486,560, Lopatin describes a method for reducing electromigration in copper by forming an interim layer of calcium-doped copper seed layer in a chemical solution. This method requires the treatment of a pre-deposited Cu seed layer in a chemical solution to form a Cu-Ca-X layer on the Cu seed layer wherein X is a contaminant. A thermal anneal and/or plasma treatment is subsequently required to reduce the contaminant levels and to form the desired Cu-Ca alloy on the surface of the Cu. Although a conformal doped film can be formed using this methodology, the same chemical bath used to introduce the Ca alloying element can also introduce contaminants detrimental to copper interconnects. The typical contaminants incorporated in the chemically treated film include carbon (C), sulphur (S), and oxygen (O) arising from the chemical solution. These contaminants can cause an increase in film resistivity and also lead to poor adhesion.
Accordingly, methods for thin film deposition that can deposit conformal films with controlled, uniform doping levels within high aspect ratio features at low deposition temperatures without the simultaneous introduction of unwanted contaminants are desirable. It is also desirable to be able to dope in controlled amounts at an atomistic scale. Therefore, despite conventional deposition techniques, there remains a need for methods and compositions that overcome the deficiencies of conventional deposition processes.